RF receiver circuits typically use narrow band filtering techniques in order to prevent undesired radio signals and spurious emissions from entering the front end amplifier of the radio. Allowing such undesired signals greatly impacts receiver performance since these undesired RF signals can cause interference with the intended mixing products within the receiver. When seeking a wider operating bandwidth for the receiver while still maintaining high performance, it is often necessary to utilize a tunable filter in order to cover a larger frequency spectrum. Tunable filters can work to optimize receiver performance by effectively tuning a narrow bandwidth filter over a wide frequency range. This gives the receiver highly selective performance since the filter can achieve the maximum attenuation in undesired frequencies which are detrimental to receiver performance.
A problem in using these types of filters is that the bandwidth and shape of the filter can vary over a given operating frequency. This occurs especially when trying to tune the filter over a wide frequency range, i.e., greater then 25 MHz. For example, FIG. 1 illustrates a schematic diagram of a well known narrow band filter used in the prior art. In operation, capacitors 101-109 form a split capacitive network which is connected to varactor diodes 115, 117. Similarly, inductors 119-123 form an inductive network which works to operate in combination with the split capacitive network to achieve filtering at some desired operating frequency. Varactor diodes 115, 117 are used with tuning voltage source VL through their respective resisters 111, 112, allowing the filter's frequency of optimal attenuation to be tuned. Thus, this allows RF signals to pass through an input port 125 of the filter to an output port 127 where a predetermined center frequency of the signal is attenuated to a desired level.
FIG. 2 is a graph illustrating return loss of the filter versus operating frequency of the filter shown in prior art FIG. 1. The frequency range depicted in the graph is approximately 260 MHz. Those skilled in the art will recognize that the bandwidth where the filter can achieve 30 dB of attenuation is only in a range of less than 20 MHz. Thus, as the frequency of the filter is tuned from its center frequency, it becomes less and less effective as it attenuates less signal. Although the filter may be tuned using its inductive network, it will only effectively tune within this very limited range. Consequently, as the filter increases bandwidth to points away from its center frequency it becomes less than optimal in achieving a desired amount of attenuation.
Hence, the need exists for a new filter topology having a sharp filter response that is tunable over a substantially wide frequency range.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.